Earth Science s 2020; 9(4): 130-142 http://www.sciencepublishinggroup.com/j/earth doi: 10.11648/j.earth.20200904.13 ISSN: 2328-5974 (Print); ISSN: 2328-5982 (Online)

Surface and Crustal Study Based on Digital Elevation Modeling and 2-D Gravity Forward Modeling in Thandiani to Boi Areas of Hazara Region,

Umair Khan 1, Fawad Khan 2, *, Tahirinandraina Prudence Rabemaharitra 1, Malik Arsalan 1, Osama Abdulrahim 1, Inayat Ur Rahman 1

1School of Geosciences & Info-Physics, Central South University, Changsha, China 2Central of Excellence in Mineralogy, University of Balochistan, Quetta, Pakistan Email address:

*Corresponding author To cite this article: Umair Khan, Fawad Khan, Tahirinandraina Prudence Rabemaharitra, Malik Arsalan, Osama Abdulrahim, Inayat Ur Rahman. Surface and Crustal Study Based on Digital Elevation Modeling and 2-D Gravity Forward Modeling in Thandiani to Boi Areas of Hazara Region, Pakistan. Earth Sciences. Vol. 9, No. 4, 2020, pp. 130-142. doi: 10.11648/j.earth.20200904.13

Received : July 25, 2020; Accepted : August 10, 2020; Published : August 19, 2020

Abstract: Gravity data indicates that there is a regular relation between crustal structure, crustal density (composition), and surface ascension. In order to delineate surface and subsurface geological structure features, and to calculate the thickness variation of the crust and sedimentary/metasedimentary wedges, integrated approach of Geographic Information System (GIS) i.e. digital elevation models (DEMs) and two-dimensional forward modeling of gravity data were utilized, which provide the best results for the primary objectives. Tectonically, the study area lies in the Lesser as well as to an extent in the sub-Himalaya, more concretely in the western limb of Hazara Syntaxis. Topographic data was accumulated in XYZ coordinates utilizing point heights method, and DEMs generation, manipulation, interpretation, and visualization process were directed to surfer-15 and ArcGIS software. Determinately the visualization of surface geological structure in the form of DEMs were proposed. The gravity stations in single contour mode have been quantified by using Scintrex CG-5 gravity meter. The collected gravity data was processed by standardizing corrections, two-dimensional forward modeling along with gravity profile were utilized and bouguer anomaly map and gravity model was computed utilizing bouguer density of 2.4 g/cm 3, where the subsurface structures are demarcated by the bouguer anomaly and gravity model. In summary this research has allowed the validation of surface and subsurface geological structure visualization. Digital elevation models provide a defensive prediction of the geological structure of the regional surface. The gravity model demarcated a series of stratigraphic units with density boundaries within the basement. The gravity model also suggests that the thickness of sedimentary/metasedimentary wedge in Thandiani area is 11.48 km and in Boi area, the thickness elongates to about 14.43 km. The total thickness of crust in Thandiani and Boi area is 49.53 km and 52.43 km respectively. Keywords: Digital Elevation Models, Gravity Model, Bouguer Anomaly, Crustal Study, Northwest Himalayas

(DEMs) can be used to recognize surface geological 1. Introduction structural trends, considering the crustal structure is closely The gravity method has numerous deals, which related to the surface elevation through isostasy phenomena. investigates anomalies caused by changes in the physical The digital elevation models (DEMs) are one of the properties of subsurface rocks and requires fundamentally fundamental spatial datasets in geographic information comparable interpretation techniques [1]. Especially gravity systems (GIS). As sharp fluctuations in elevation is a sign of surveys have been widely used for crustal study, subsurface changes in the geological structure. Commonly, modeling structure characterization and visualization, and proved to be surface structure can partly uncover the depth structure of the a useful tool [2]. In addition, digital elevation modeling exploration target [3-6]. Crustal thickness diversifies Earth Sciences 2020; 9(4): 130-142 131

significantly across continental margins, knowledge of the is suggested that the shield elements with increasing location of the crust-mantle interface (Moho), and the sediment cover can be followed by the northward movement structure of the crust is imperative for understanding and underneath the southern thrust of Himalayas [16, 17]. evaluation continental rifting processes, geological evolution, The Thandiani to Boi area of Hazara region lies in the western and modern tectonic conditions. Gravity data usually limb of Hazara Kashmir Syntaxis (HKS) which is highly folded investigate a regular relationship between crustal structure, and faulted and developed due to the stresses affected by the crustal density (composition), and surface elevation. collision of Indian and Eurasian plates [18]. The present study Geodetic computation has been made in the Himalayas for area bounded by latitude 34°18′600″ N and longitude over 125 years. It has been great importance in the 73°26′11.99′′ E (Figure 1). This study is predicated on the ascertainment of the deep structure of the earth’s crust [7, 8]. integrated methods of digital elevation modeling (DEMs) and Gravity anomalies have been widely used to study the two-dimensional forward modeling of gravity data, proposing to northwest Himalayan crustal configuration [8-13]. In the identify the surface and subsurface structure trends to calculate northern Pakistan, gravity measurement has been done the thickness of the crustal and sedimentary/metasedimentary principally by various researches [10, 12]. Malinconico has wedge of the study area. Predicated on two-dimensional forward conducted gravity and magnetic survey across the Main modeling method gravity anomaly was computed using Mantle Thrust (MMT) and Main Karakoram Thrust (MKT); GM-SYS profile modeling from Bouguer anomalies map of He was the first researcher who interpreted Thandiani to Boi area of Hazara region, two-dimensionally the terrain-corrected gravity data. He (KPK) Pakistan. Projected gravity data have been practiced to suggested that the Indian plate is being thrust under the generate a gravity model to illustrate the relationship of known Eurasian plate and that the primary structure and this region geology with the gravity anomaly. The model parameters (e.g., dip towards the north. The northern structure being older, dip dimensions, density, and depth) are adjusted until the best fit steeper than those of the south, which are younger [14, 15]. between the calculated gravity and observed anomalies were Farah analyzed the Pre-Cambrian basement of the Punjab achieved. plain and Potwar plateau by gravity modeling in the south. It

Figure 1. Location map of the study area (Thandiani to Boi), Hazara region Pakistan, with location of gravity profile (D-D/). 132 Umair Khan et al. : Surface and Crustal Study Based on Digital Elevation Modeling and 2-D Gravity Forward Modeling in Thandiani to Boi Areas of Hazara Region, Pakistan

2. Geological and Regional Tectonic in Figure 1 and Table 1. Several rock samples were collected from the study area, and their bulk density was measured in Setting the laboratory (Table 2).

2.1. Geological Setting 2.2. Regional Tectonic Setting

The regional geology of the predicted region consists of Geologically Pakistan's Himalayan organic belt has several sedimentary, metamorphic, and igneous rocks, which differed unique geological features. The central section implements a from Precambrian to recent (Table 1). The geological simple profile of transcontinental-continental collision succession with a concise explanation of several stratigraphic boundaries (Figure 3). Along its 2500 km long structure begins units of the study area is given in Figure 2, which is approved with a remarkable regular series, so there is an exceptional by the Stratigraphic committee of Pakistan [18, 19]. The possibility to study variations along the length of the structure, basement rocks of the study area are not in the typical the tectonic belt, the volcanic plutonic arc, the island arc, and sequence and are separated by different faults. The thrust the length of the thrust belt (Figure 3). For all these purposes system of the East and West limbs of the Hazara Kashmir and more, the Himalayas contribute a good prospect of Syntaxis (HKS) was developed due to the collision of the geological phenomena [21]. The Himalayan range was created Indian plate and the Eurasian plate. The Kashmir Boundary by the collision between the Indian and Eurasian plates [19]. Thrust (KBT) is the youngest structural feature of the The northern boundary of the Eurasian and Indian plates is an projected area, which cuts the Main Boundary Thrust (MBT), ophiolitic belt known as the Indus-Tsangpo Suture Zone (ITSZ) Panjal Thrust (PT), and Jhelum Fault (JF) near the apex [19]. [22]. The ITSZ, which extending westward, bifurcated into two Most of the study area is incorporated by sedimentary rocks; major thrust faults and surrounded a sequence of rocks. The however, metamorphic and igneous rocks are also existed MMT and Main Karakoram Thrust (MKT). Kohistan Island [20]. For two-dimensional gravity forward modeling, the Arc is present between these thrust faults. The northern suture geology of the region is divided into six units, which are zone that is MKT separates the intrusive high-grade formation of Miocene age (sandstone, mudstone, and metamorphic rocks of the Eurasian Plate from the Kohistan siltstone), Kuldana formation of Eocene age (shale and marl), terrain, whereas the southern suture MMT separates the carbonates from Cambrian to Eocene age (shale, limestone, Kohistan Island Arc from the Meta sediments of the northern and sandstone), Tanol formation of Precambrian age (pelitic margin of the Indian Plate. The Island arc terrain is composed of and psammitic metasedimentary rocks with local subordinate a sequence of high-density rocks, while the rocks of Indian and graphitic schist, talc schist, and marbles), Hazara formation Eurasian plates are of types of lower density [23]. of Precambrian age (slates and phyllites), and Salt Range formation of Precambrian age, these all information is given

Figure 2. Simplified geological map compiled after [14], the blue points indicates gravity stations, and red point represent the study region. Earth Sciences 2020; 9(4): 130-142 133

Figure 3. Regional tectonic configuration of Northern Pakistan along with the study area after [11, 13]. The inset shows the location of the study region.

Table 1. Regional geological succession, after [18].

Formation Lithology Age Alluvium Unconsolidated deposits of sand, silt, clays, and gravels Recent Murree Formation Clays, shales and sandstone, Miocene Unconformity Kuldana Formation Shale with clay, calcareous and gypsum-bearing beds Middle Eocene Chorgali Formation Marly, creamish, and thinly bedded dolomitic limestone Upper Eocene Margala Hills Limestone Well-bedded nodular limestone Eocene Patala Formation Khaki-colored shales with subordinate limestone and Marly beds Palaeocene Lockhart Limestone Nodular limestone with subordinate shales Palaeocene Hangu Formation Laterite, bauxite, carbonaceous shales, coals, limonite, quartzite Palaeocene Unconformity Kawagarh Formation Well-bedded marl and limestone Upper Cretaceous Lumshiwal Formation Ferruginous brownish glauconitic sandstone with shales Cretaceous Chichali Formation Blackish splintery shales with subordinate sandstone Cretaceous Unconformity Samanasuk Formation Gray, yellow color well-bedded oolitic dolomitic limestone Jurassic Datta Formation Sandstone, quartzite (in Hazara region, most are quartzite) Jurassic Unconformity Hazara Formation Glauconitic ferruginous shale and sandstone Cambrian Abbotabad Formation Boulder sandstone quartzite thick bedded limestone and phosphate Cambrian Mansehra Granite Porphyritic granite with phenocryst and xenoliths Cambrian Unconformity Tanol Formation Quartzite, phyllite, sandy facies, and schistose conglomerate Pre-Cambrian Hazara Formation Slates, phyllite, with minor occurrence of limestone and gypsum Pre-Cambrian 134 Umair Khan et al. : Surface and Crustal Study Based on Digital Elevation Modeling and 2-D Gravity Forward Modeling in Thandiani to Boi Areas of Hazara Region, Pakistan

Figure 4. The methodology adopted for construction of 2D/3D Digital Elevation Models, and gravity model.

grid data structure, only the elevation (z) of each node of the 3. Materials and Methods grid is recorded. All fluctuations of the terrain cannot be The components of the methodology section can be divided covered by the cell size of the mesh [24]. Therefore, we utilize into three parts, namely, digital elevation modeling (DEMs), The TIN structure to examine the surface structure changes in bouguer anomaly mapping, and two dimensional gravity data the study area. In the TIN data structure, the surface elevation forward modeling (Figure 4). Conventional preparation is X, Y, and Z of the terrain specific point are recorded, which imperative for any sort of geophysical and geographical describes a more realistic structure. There are numerous investigations. Before starting this comprehensive techniques to accumulate the DEM data, i.e., point height, investigation, at our deskwork, we studied previous research stereoscopic images, active remote sensing methods (radar, to gather information about the geological and geophysical lidar) [25]. This study practices the point-height method conditions of the study area. The present study appropriates because its high-resolution images and their characteristics, various geological maps, topographic maps, remote sensing such as real-time data, temporal data, and the accuracy of data, gravity data, and additional sources of information that horizontal and vertical measurements are most popular. First, could be beneficial for surface and subsurface structure a two dimensional and three-dimensional digital elevation modeling. The present study utilizing integrated software’s model based on elevation data was assembled for Thandiani such as GM-SYS, Surfer15, and ArcGIS. The primary and Bio region. Subsequently, two-dimensional and consideration of gravity investigation was the establishment three-dimensional DEMs were produced for the regional zone. and modeling of base stations and auxiliary stations in the Terrain data were accumulated in XYZ coordinates from research area. The D-D/ profiles were usually selected based Google Earth, and DEM generation, DEM manipulation, on the main geological structures in the Thandiani to Bio area, DEM Interpretation, and DEM Visualization process were the Hazara region of Pakistan. directed to Surfer15 and ArcGIS software (Figure 4). A DEM based on contours consists of a set of contours that are equal to 3.1. Digital Elevation Models (DEMs) the corresponding elevation value, "Mark". We use a 50-meter interval between profiles. The contours were stored in ordered The digital elevation model is a digital illustration of coordinates and can be thought of as polylines or polyline arcs three-dimensional information (X,Y,Z) of the continuous containing elevation values. topography of the bare earth surface in a particular reference coordinate system [4]. The surface geological structure of the 3.2. 2D Gravity Forward Modeling Thandiani to the Boi region in the Hazara region is considerably complicated. In this study, the analysis of bare 3.2.1. Data Acquisition terrain was conducted based on the DEMs, as dramatic The relative gravity meters CG-5 Scintrex with a perceived fluctuations in elevation are signs of variations in the resolution of 1µGal were used for gravity data acquisition geological structure [5]. There are ordinarily two primary data (Figure 5). Three GPS receivers were used to obtain each structures, the grid structure and the TIN (triangular irregular stations location coordinates (Table 3), including TOPCON's network) structure, where DEMs data can be stored. In the GRS-1 type and the 5700 L1 type of two TRIMBLERs. The GRS-1 utilizes dual frequencies, while the 5700 L1 uses one Earth Sciences 2020; 9(4): 130-142 135

frequency to obtain location information [26]. This information 3.2.2. Data Processing helps us to understand the structural trends in our field of study. The main purpose of data processing is to improve the The dataset was executed in a single profile mode, and the main observation signal-noise ratio to achieve a reliable and auxiliary base stations were established. The main base explanation [29]. By applying the standard correction to each station for gravity measurement was built in the campus of Azad station's measurements, the original gravity data were Jammu University. It was connected to the National Base reduced to produce the complete Bouguer anomaly given in Station of the Church of Christ Lal Kurti Rawalpindi. The equation (1). For obtained data, all changes in the Earth's gravity of the Rawalpindi National Base Station (RNBS) was gravitational field must be corrected, not due to differences in beforehand connected with the Teddington and Washington the density of the underlying rock (Figure 6). In order to Pendulum Stations [27]. The auxiliary base stations were calculate and interpret the observed gravity data, it is established in Boi. As a consequence, all measurement essential to determine the density and topographic correction dimensions have been incorporated into the International of the subsurface rock units [29]. Several authors examine Gravity Network (IGN) [28]. The gravity value of the base different methods for weight reduction density determination station is shown in Table 3. Since the study persistence was to for gravity data. Utilizing the F-H method proposed by delineate the subsurface geological structure more clearly, it was Parasnis (Figure 6), the method is based on the minimum decided to conduct a single profile gravity survey of the correlation between Bouguer gravity anomaly and Bouguer geological structure in the design area. Meanwhile regional correction [30]. The reduction density of 2.4g/cm 3 is gravity measurements, the interval between stations are usually determined to be the gradient of the line (Figure 2) to correct administered 1 km, along a single section. the gravity data. Table 2 shows the absolute gravity value, free air anomaly, and the range of values for the complete Bouguer anomaly. The ultimate goal of the reduction in gravity data is to produce a complete Bouguer anomaly. Based on the above procedure, the Bouguer anomaly is given by the following equation:

∆g = g obs –γ+βh-2πGρh-ρT (1)

Where “∆g” shows Bouguer anomaly in m Gal , ‘g obs ” represent observed gravity value (mGal ), “γ” shows normal gravity (mGal ), “βh” represent free-air correction (m Gal ), “h” is the Elevation (m), “G” represents gravitational constant (6.67×10-11m 3kg -1s-2), “ρ” is the estimated density (g/cm 3), “2πGρh” show bouguer correction (mGal ), and “ρT” represent Figure 5. CG-5 Autograv utilized for gravity data acquisition. terrain correction value (mGal ).

Figure 6. Calculation of Bouguer density using Parasnis approach (F-H method). 136 Umair Khan et al. : Surface and Crustal Study Based on Digital Elevation Modeling and 2-D Gravity Forward Modeling in Thandiani to Boi Areas of Hazara Region, Pakistan

Table 2. Density used for different formation in the study area, after [23].

Formation Lithology Density (gm/cc) Average Density (gm/cc) Siwaliks group Sandstone and Shale 2.35-2.55 2.45 ±0.1 Muree formation Sandstone and Shale 2.44-2.66 2.55 ±0.11 Carbonates rocks Limestone 2.60-2.75 2.67 ±0.08 Hazara formation Slates and Shale 2.48-2.55 2.53 ±0.04

is carried out along selected D-D/ profile by utilizing the 3.2.3. Data Interpretation Paranis approach (Figure 6), which combines previously The Bouguer anomaly is caused by the heterogeneity of the known structural and geological information in the study area distribution of density in the subsurface. Once the gravity data [28]. This method was used to calculate exceptions. The is reduced to the Bouguer anomaly, the next step usually observation of the gravity values along this segment were involves meshing the data to generate a map, followed by taken from the Bouguer anomaly map. According to the interpretation [31]. Therefore, the interpretation of gravity data formation characteristics and density response, the is based on the Bouguer anomaly. Various geological structures subsurface layer is divided into different parts, namely, are approximately linear and can be solved by alluvial, Murree formation, carbonate, Hazara formation, salt two-dimensional interpretation. Different authors discussed the range formation, and crystal crust. The density values of steps involved in interpreting gravity anomaly data [32]. In this several rocks were perceived (Table 2). The computational study, two-dimensional forward modeling were used to exhibit error range of our model is from 0.54 to 0.773, which is the basements deep subsurface structure. mathematically sensible. In the profile, the basement section 3.2.4. 2D Forward Modeling shows different formations included in the model, their The GM-SYS profile of Geosoft Oasis Montaj 8.0.1 respective property densities during the modeling process. software was used for the interpretation of gravity data. The We select different colors to distinguish the formations GM-SYS profile is a program for determining the gravity and following the information for the basement from the magnetic response of geological cross-sections. Forward geological with assumed density. In the gravity modeling, the modeling includes building hypothetical geological models density values are adjusted until a reasonable fit is achieved and assessing geophysical responses [32]. Gravity modeling between the observed calculated gravity anomalies. Table 3. Parameters of National, Main, and Auxiliary base stations in the study area.

Stations Gravity (Mgals) Elevation (meters) Latitude Longitude National Base Station (Rawalpindi) 979268.43 511 33°58′60″N 73°03′34″E Main Base Station (UAJK) campus 979282.38 743 34°22′25″N 73°28′10″E Auxiliary Base Station (Bio area) 979280.141 816 34°18′24″N 33°58′60″E

accuracy). Various factors impersonate an important role in the 4. Result and Discussion quality of DEM-derived products i.e., terrain roughness, sampling density (elevation data collection methods), grid 4.1. Digital Elevation Models resolution or pixel size, interpolation algorithm, vertical resolution, and terrain analysis algorithm [34]. The efficiency of Prior to the appearance of remote sensing and digital using a DEM for surface geological structure characterization is elevation models, field geologists were only capable of demonstrated here by the agreement between previous surface obtaining large-angle views of specific geological areas through geological study and DEM derived data. The advantage of such geological maps [33]. In areas with reduced outcrops, field a method is to give an interpretation at the scale of the slope, surveys often provide local observations that make it difficult to which is not possible when carrying out fieldwork at the infer and incorporate more regional structures. With the regional outcrop scale. The result shows that analysis based on availability of geographic maps and visualization tools such as DEM allows identification of large scale structure and surface ArcGIS Pro, Google Earth Pro, and Surfer 13, it is reasonable to of failure and regional tectonic features. The above analysis check large areas [6]. Global remote sensing mosaics with shows how the use of DEM can help obtain the first sub-resolution, such as Google and Bing Web services, can interpretation of instability of surface geological and terrain broadly identify and depict geological structures. In this section, structure. The surface structural trend of Thandiani and Boi area DEMs are commonly built using topographic data, predicated is presented in Figure 7 and Figure 8. The regional topographic remote sensing techniques. The quality of DEMs is a measure surface map is presented in Figure 9. of how accurate elevation is at each pixel (absolute accuracy) and how accurately the morphology is presented (relative

Earth Sciences 2020; 9(4): 130-142 137

Figure 7. Digital elevation model (DEM) of Thandiani area, showing topographic structural trend.

Figure 8. Digital elevation model (DEM) of Boi area, showing topographic structural trend. 138 Umair Khan et al. : Surface and Crustal Study Based on Digital Elevation Modeling and 2-D Gravity Forward Modeling in Thandiani to Boi Areas of Hazara Region, Pakistan

Figure 9. Topographic structural trend at the regional scale of the study area.

The size and shape of the anomaly imply that it likely resulted 4.2. Bouguer Anomaly Map from complex source structures. Due to the composition and The Bouguer anomaly map in the projected area is compiled structure of the earth's crust, gravity anomalies vary from place in the 1:500,000 range with a section interval of 2 mGal. The to place. The purpose of crustal deformation measurement is to Bouguer anomaly map exhibits the presence of different determine the relative position or gravity changes of points on anomaly zones in the west and north of the studied area. The the earth's surface, to achieve geometric and physical gravity values of the Thandiani to the Boi region range from information about crustal deformation; the concept of measurement is displacement, and the relative change of strain -124 M gal to -234 M gal (Figure 10. b). In general, high and low and gravity values [35]. The Bouguer gravity reduction density gravity anomalies were perceived in the northwest and 3 southwest regions of the study area, respectively. These was reduced with 2.67 g/cm . The contours on the gravity changes in the Bouguer anomaly map reveal the heterogeneity anomaly map usually trend northwest to the southwest, and of the distribution of subsurface density. The anomaly is gravity descends to the northeast. The trend of bearing force in characterized by mutually alternating local maxima and minima. the study area indicates that the thickness of the crust is Earth Sciences 2020; 9(4): 130-142 139

increasing towards the northeast. According to the Bouguer deformation is observed in the rock sequence of the surface. In anomaly contours, the area is divided into three regions, the central area between Thandiani and Boi, gravity values are northwest, central, and southwest. In the southwest of the relatively changing. In the Northwest Boi region, gravity Thandiani region, the area is relatively stable, with fewer anomalies increased, indicating changes in crustal thickness. changes in gravity shown on the map. In this area, less

(a)

(b) Figure 10. Regional geological map (a) with Bouguer anomaly map along with the selected profile D-D/ (b) for gravity modeling. 140 Umair Khan et al. : Surface and Crustal Study Based on Digital Elevation Modeling and 2-D Gravity Forward Modeling in Thandiani to Boi Areas of Hazara Region, Pakistan

4.3. 2D Gravity Model along the D-D′ Profile provided by deep-source gravity anomalies. According to the literature, density values have been assigned to non-survey The two-dimensional gravity modeling (along profile D-D′ parts of the subsurface. The density distribution is gradually selected from Thandani to the Boi area) was carried out to a altered to achieve the best fit between observing gravity and depth of 54 km employing software-based on the method of calculating gravity [28]. Given the primary constraints, the Talwani and Paranis approach [32, 35]. As a result the gravity best fit between the measured and observed anomalies can be model were constructed which reveals that the thickness of the accomplished by assigning a slightly lower density to the crust increases to the northeast in the study area. However, in south-west and north-east subsurface. For calculating and the north-east, KBT is divided between the observing mutations in gravity data, modeling is based on formation and the Muree formation of molasses. Following the three steps, namely sediment, Moho effects, and the joint gravity model, the above part shows a graph between the effects of Moho and sediment on the 38 km thick Indian shield observed gravity values (pink) and the calculated gravity crystal crust [36]. The density ratio assigned to the mantle and values (green) (Figure 11). The Y-axis exhibits gravity in Mgals geological bodies was 2.95 g/cm 3 compared to the average and the X-axis in meters. The calculated model explicates that a density of crystal basements. In comparison, the density thin layer of salt is present in the subsurface that pinches out assigned to the mantle was 3.30 g/cm 3. toward the northeast of the study area. Following the result of Concerning the shape of sedimentary/metasedimentary 2D gravity modeling, it is suggested that the normal crustal wedges, the most conservative solution means simplified thickness in the southwest part (Thandiani area) has been plane geometry. Finally, the low gravity in the Thandiani to assumed to be 49.53 km. In the northwest part (Boi area) is Boi area compensates with the effects of regional density 52.43 km. The thickness of the sedimentary/metasedimentary discontinuity (e.g., Moho and the top of the crystal basement). wedge in the Thandiani area is 11.48 km. In the Boi area, the The density contrasts of the subsurface vary, such as thickness extends to about 14.43 km (Figure 11). (Alluvium, Muree formation, carbonates, Hazara formation, In the gravity model, the geological structure approximates Salt Range formation, crystalline crust, and Moho) have -0.8, a horizontal prism with a finite length and polygonal -0.45, -0.35, -0.48, -0.7, 0,0.35 gravity values, respectively. cross-section. The model takes into account the constraints

Figure 11. Gravity model represents the deposition and Moho effects as well as the profile D-D/. Profile starting from the southwest Thandiani area and ending at the northwest Boi area, which connect the gravity data line. Densities are in Mgal.

of surface geological and terrain structure by utilizing digital 5. Conclusion elevation models (DEMs), (2) the evaluation and This study discusses: (1) The interpretation of uncertainty characterization of subsurface geological structure features, Earth Sciences 2020; 9(4): 130-142 141

basement configuration, thickness variation of crust and [7] Andersen OB, editor The DTU10 Global Gravity field and sedimentary/metasedimentary wedges and to comprehend the mean sea surface–improvements in the Arctic. 2nd IGFS meeting; 2010. relationship of known geology with the gravity anomaly. Firstly surface topographic structure was described employing [8] Banjeree P, Satyaprakash W. Crustal configuration across the Geographic Information System (GIS), i.e., DEMs. Terrain north-western Himalaya as inferred from gravity and GPS data were accumulated in XYZ coordinates from Google aided geoid undulation studies [J]. Journal of the Virtual Explorer. 2003; 12: 93-106. Earth, DEMs generation, DEMs manipulation, DEMs interpretation, and DEMs visualization process were directed [9] Caporali A. Gravity anomalies and the flexure of the into Surfer15 and ArcGIS software. Subsequently, two and lithosphere in the Karakoram, Pakistan. Journal of Geophysical three-dimensional color embossed shaded relief DEMs were Research: Solid Earth. 1995; 100 (B8): 15075-85. constructed, which exhibit topographic structural features, [10] Bai Z, Zhang S, Braitenberg C. Crustal density structure from allowing them to identify a surface geological structural trend 3D gravity modeling beneath Himalaya and Lhasa blocks, associated with the subsurface structure. Secondly, the Tibet. Journal of Asian Earth Sciences. 2013; 78: 301-17. interpretation of gravity data was performed using [11] Khan MR, Bilali SS, Hameed F, Rabnawaz A, Mustafa S, two-dimensional density forward modeling employing the Azad N, et al. Application of gravity and magnetic methods GM-SYS software. Bouguer anomaly map depicted the for the crustal study and delineating associated ores in the crustal variations across the study area, and different western limb of Hazara Kashmir Syntaxis, Northwest formations along their densities were identified. Bouguer Himalayas, Pakistan. Arabian Journal of Geosciences. 2018; 11 (6): 131. anomaly map is decreasing gradually towards the northeast due to an increase in crustal thickness. Less deformation is [12] Lyon-Caen H, Molnar P. Gravity anomalies, flexure of the observed in the subsurface rock sequence. In the Northwest of Indian plate, and the structure, support and evolution of the the study area, gravity anomalies increased, indicating Himalaya and Ganga Basin. Tectonics. 1985; 4 (6): 513-38. changes in crustal thickness. The gradual thickening of crust [13] Butt AA. Problems of stratigraphic nomenclature in the Hazara towards the northeast of the study area is associated with the District, NWFP, Pakistan. Geol Bull Punjab Univ. 1972; 9: collision of Indian and Eurasian plates. A thin layer of salt is 65-9. present in the sub-surface that pinches out toward the [14] Calkins J, Offield T, Ali S. Geology and mineral resources of northeast of the study area. Based on the two-dimensional southern Hazara district, West Pakistan and part of western computed gravity model, the thickness of the Kashmir. United States Department of State and Government sedimentary/metasedimentary wedge in the Thandiani area is of Pakistan project report. 1969; 43: 92. 11.48 km. In the Boi area, the thickness extends to about 14.43 [15] Caporali A. The gravity field of the Karakoram Mountain km. The total thickness of the crust in the Thandiani and Boi Range and surrounding areas. Geological Society, London, area is 49.53 km and 52.43 km, respectively. Special Publications. 2000; 170 (1): 7-23. [16] Greco AM. Tectonics and metamorphism in the western Himalayan syntaxis area (Azad Kashmir, NE-Pakistan): ETH References Zurich; 1989. [1] Braitenberg C, Zadro M, Fang J, Wang Y, Hsu H. The gravity [17] Umar M, Sabir MA, Farooq M, Khan MMSS, Faridullah F, and isostatic Moho undulations in Qinghai–Tibet plateau. Jadoon UK, et al. Stratigraphic and sedimentological attributes Journal of Geodynamics. 2000; 30 (5): 489-505. in Hazara Basin Lesser Himalaya, North Pakistan: their role in deciphering minerals potential. Arabian Journal of Geosciences. [2] FUKAO Y, YAMAMOTO A, NOZAKI K. A method of 2015; 8 (3): 1653-67. density determination for gravity correction. Journal of Physics of the Earth. 1981; 29 (2): 163-6. [18] Shah SI. Stratigraphy of Pakistan. 1977. [3] Burrough PA, McDonnell R, McDonnell RA, Lloyd CD. [19] Iqbal S, Nasir S, Hussain A. Geological map of the Nauseri Principles of geographical information systems: Oxford area, District Muzaffarabad, AJK: Geol. Survey of Pakistan university press; 2015. Geological Map Series. 2004; 6 (14).

[4] Jenson SK, Domingue JO. Extracting topographic structure [20] Gee E, Gee D. Overview of the geology and structure of the from digital elevation data for geographic information system Salt Range, with observations on related areas of northern analysis. Photogrammetric engineering and remote sensing. Pakistan. Geological Society of America special paper. 1989; 1988; 54 (11): 1593-600. 232: 95-112. [5] Onorati G, Ventura R, Poscolieri M, Chiarini V, Crucilla U. [21] Zahid M, Ahmad S, Ali F, Rehman G. Structural geometry of a The digital elevation model of Italy for geomorphology and part of the southeastern Hazara Fold-Thrust Belt, Khyber structural geology. Catena. 1992; 19 (2): 147-78. Pakhtunkhwa, Pakistan. Pakistan Journal of Hydrocarbon Research. 2009; 19. [6] Webster TL, Murphy JB, Gosse JC, Spooner I. The application of lidar-derived digital elevation model analysis to geological [22] DiPietro JA, Pogue KR. Tectonostratigraphic subdivisions of mapping: an example from the Fundy Basin, Nova Scotia, the Himalaya: A view from the west. Tectonics. 2004; 23 (5). Canada. Canadian Journal of Remote Sensing. 2006; 32 (2): 173-93. [23] Calkins JA, JA C, TW O, SKM A. Geology of the southern Himalaya in Hazara, Pakistan and adjecent area. 1975. 142 Umair Khan et al. : Surface and Crustal Study Based on Digital Elevation Modeling and 2-D Gravity Forward Modeling in Thandiani to Boi Areas of Hazara Region, Pakistan

[24] Spencer JE. Structural analysis of three extensional detachment [30] Afonso J, Ranalli G, Fernandez M. Density structure and faults with data from the 2000 Space-Shuttle Radar buoyancy of the oceanic lithosphere revisited. Geophysical Topography Mission. GSA Today. 2010; 20 (8): 4-10. Research Letters. 2007; 34 (10). [25] Silva CL, Morales N, Crósta AP, Costa SS, Jiménez-Rueda JR. [31] Casten U, Snopek K. Gravity modelling of the Hellenic Analysis of tectonic-controlled fluvial morphology and subduction zone-a regional study. Tectonophysics. 2006; 417 sedimentary processes of the western Amazon Basin: an (3-4): 183-200. approach using satellite images and digital elevation model. Anais da Academia Brasileira de Ciências. 2007; 79 (4): [32] Bhattacharyya B. Computer modeling in gravity and magnetic 693-711. interpretation. Geophysics. 1978; 43 (5): 912-29. [26] Wang C-y, Lou H, Wei X-c, Wu Q-j. Crustal structure in [33] Chen N, Ni N, Kapp P, Chen J, Xiao A, Li H. Structural analysis northern margin of Tianshan mountain and seismotectonics of of the Hero Range in the Qaidam Basin, northwestern China, the 1906 manas earthquake. Acta Seismologica Sinica. 2001; using integrated UAV, terrestrial LiDAR, Landsat 8, and 3-D 14 (5): 491-502. seismic data. IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing. 2015; 8 (9): 4581-91. [27] Woollard G. Crustal structure from gravity and seismic measurements. Journal of Geophysical Research. 1959; 64 (10): [34] Blythe A, Burbank D, Farley K, Fielding E. Structural and 1521-44. topographic evolution of the central Transverse Ranges, California, from apatite fission-track,(U-Th)/He and digital [28] Bernabini M, Cifelli F, Di Bucci D, Funiciello F, Orlando L, elevation model analyses. Basin Research. 2000; 12 (2): 97-114. Parotto M, et al. Studio gravimetrico 3D dell’Italia centrale. Atti del. 1996; 15: 49-52. [35] Adighije C. A gravity interpretation of the Benue Trough Nigeria. Tectonophysics. 1981; 79 (1-2): 109-28. [29] Bhattacharyya B, Chan K. Computation of gravity and magnetic anomalies due to inhomogeneous distribution of [36] Talwani M, Worzel JL, Landisman M. Rapid gravity magnetization and density in a localized region. Geophysics. computation for two-dimensional bodies with application to the 1977; 42 (3): 602-9. Mendocina submarine fracture zone. Journal of geophysical research 1959; 64 (1): 49-59.